The virus, xenotropic murine leukemia virus-related virus or XMRV, was
detected in 67% of 101 patients with chronic fatigue syndrome by Vincent C.
Lombardi, PhD, of the Whittemore Peterson Institute in Reno, Nev., and
colleagues.

The researchers also found the virus in nearly 4% of healthy comparison
subjects -- suggesting that millions of Americans may carry the mysterious
virus, which was first detected in prostate cancers.

"The discovery of XMRV in two major diseases, prostate cancer and now chronic fatigue syndrome, is very
exciting. If cause and effect is established, there would be a new opportunity
for prevention and treatment of these diseases," said Robert H. Silverman, PhD,
of Cleveland Clinic's Lerner Research Institute, in a statement emailed to
WebMD.

Silverman is on of the team of scientists that first discovered XMRV, and
was among the researchers linking the virus to chronic fatigue syndrome and
prostate cancer.

In prostate cancer patients, the virus is seen in patients who carry a genetic mutation that disables a key virus-fighting immune response. But the
virus is seen in chronic fatigue patients with and without this mutation.

Where did the virus come from? The virus is closely related to a retrovirus
that's become part of the mouse genome. Oddly, XMRV cannot infect mouse cells
-- but can easily infect human cells.

It's unlikely that so many humans have caught XMRV from mice. It's more
likely that the virus is spread from human to human, but how that happens
remains to be seen.

An editorial by John M. Coffin of Tufts University, Boston, and Jonathan P.
Stoye of the Institute for Medical Research, London, accompanies the Lombardi
report in the current issue of the online journal Sciencexpress.

Coffin and Stoye note that if 4% of healthy people truly do carry XMRV, it
means that the virus is astonishingly widespread.

"If these figures are borne out in larger studies, it would mean that
perhaps 10 million people in the United States and hundreds of millions
worldwide are infected with a virus whose pathogenic potential for humans is
still unknown," they write.

What is known is that viruses closely related to XMRV do cause many
different diseases -- including cancer -- in other warm-blooded animals.

"Further study may reveal XMRV as a cause of more than one well-known 'old
disease,' with potentially important implications for diagnosis, prevention,
and therapy," Coffin and Stoye suggest.

SOURCES: Lombardi, V.C. Sciencexpress, Oct. 8, 2009.

Coffin, J.M. and Stoye, J.P. Sciencexpress, Oct. 8, 2009.

Urisman, A. PloS Pathogens, March 2006.

Schlaberg, R. Proceedings of the National Academy of Sciences, published online before print, Sept. 9, 2009.